Active noise cancellation involves superimposing on a noise acoustic wave an opposite acoustic wave that destructively interferes with and cancels the noise wave. The active noise cancellation principle is most useful at predetermined frequencies in the active noise cancellation range.
In active noise cancellation systems the characteristics of the noise acoustic wave are sensed, a cancelling acoustic wave is produced and delivered to a location through a speaker. The combined waves are monitored at the location and a feedback or error signal is produced for iterative adjustment of the cancelling acoustic wave.
Implementations of the active noise cancellation principle are arranged to accommodate changes in the frequency and intensity characteristics of the noise acoustic wave by incorporating adaptability into the feedback or error path of the active noise cancellation system. The changes are accommodated through iterative incremental computations, based on the noise acoustic wave input signal and the error signal, in a procedure known in the art as an algorithm, which in turn is implemented through digital signal processing (DSP) in semiconductor chip active noise controller devices.
A number of algorithms with adaptability that are suitable for digital signal processing have evolved in the art. A survey article by J. C. Stevens entitled "An Experimental Evaluation of Adaptive Filtering Algorithms for Active Noise Control", Georgia Institute of Technology, GRTI/AERO, Atlanta, Ga. 1992 Pages 1-10, provides an illustrative description of the current capabilities in the art.
The active noise cancellation principle has been applied extensively in the art where the system can be constructed so that the noise source is localized and the operations of sensing, cancelling and monitoring can occur serially, as in ducts and pipes. An illustrative example is U.S. Pat. No. 4,987,598.
Active noise cancelling systems exhibit instability when the cancelling signal gets into the noise acoustic wave prior to the sensing of the characteristics of the noise acoustic wave. Heretofore in the art this has been handled by care in constructing a system to prevent the situation and to some extent by modification of the algorithm to accommodate it.
Situations are being encountered, particularly in such places as vehicles, where the use of an indirectly sensed signal such as from a tachometer or accelerometer that is representative of the noise acoustic wave would be useful. Examples of such situations are: where there is limited flexibility in arranging a system to prevent a cancelling signal from reaching a direct acoustic input; and where particular types of sounds such as music and warning signals should not be cancelled.
An example of early effort for vehicles is an article by Perry et al entitled "The Use of DSP for Adaptive Noise Cancellation for Road Vehicles", Paper No. 3, Session 3, Pages 331 to 338, in which tachometer or ignition based indirect sensing of the acoustic noise is processed in a controller to provide a cancelling signal for sound in an entire multi occupant enclosure through the use of a plurality of peripherally mounted speakers with monitoring through microphone pairs at each occupant seat.
A principal problem with indirect signal sensing has been that the indirectly sensed signal, while related to the noise acoustic wave that is to be cancelled, is not correlated closely enough to it to contain all the characteristics essential to efficient algorithm computations and effective noise cancellation. Recent efforts in the art avoid the problem by having table look up arrangements that use the indirectly sensed signal to guide the arrangement. Examples are U.S. Pat. 4,506,380 Nos. and 5,146,505.
A need is present in the art for the ability to correlate an indirectly sensed signal representative of an actual noise acoustic wave with the essential aspects of that actual noise acoustic wave and for a system of using direct and indirect sensing.